1. Field of the Invention
The present invention relates to an organic EL display which is formed by using, particularly, a plastic substrate and is driven by a thin film transistor including an active layer containing a non-metallic element and a method of manufacturing the organic EL display.
2. Description of the Related Art
Recently, as development of technologies for materials, manufacturing, driving circuits, and the like, a technology for electroluminescence (EL) has been put to practical use of an organic EL display as one of flat panel displays (FPDs).
In 1997, a monochromatic organic EL display was put to practical use. After that, area colorization of the organic EL display is implemented, so that the use thereof is expended to displays such as a small-size audio apparatus or a mobile terminal. In 2001, the colorization was put to practical use by a passive-matrix-type color display for a mobile phone. After that, an active-matrix-type colorization has been made by using thin film transistors. In 2007, the organic EL display was adopted to 11-inch TVs. Recently, large-size TVs having 40 inches or more has been developed.
In an organic EL device configuring an organic EL display, an electron injection layer, an electron transport layer, a light emitting layer, a hole transport layer, a hole injection layer, and the like are stacked between positive and negative electrodes, and a current is allowed to flow in the organic EL device by applying a voltage between the positive and negative electrodes, so that light is emitted. A plurality of display pixels constructed with the organic EL devices are two-dimensionally arranged and used as a display.
For the colorization of an organic EL display, various methods such as a paint division method, a color conversion method, a micro-cavity method, or a color filter method have been proposed. Among these methods, the paint division method and the color filter method are representative methods.
In the paint division method, a display pixel is divided into a plurality of pixels, which are called sub-pixels as devices for emitting light of red (R), green (G), and blue (B). Four colors including white (W) and the three colors of RGB may be used for the sub-pixels.
In the color filter method, white light is emitted, and an RGB color filter is combined with the sub-pixels, so that the colorization is implemented. Similarly to the paint division method, the color filter for the sub-pixels may include four colors of white (W) and the RGB.
The organic EL device is a surface-shaped self light-emitting device made of a totally solid state material. In comparison with a liquid crystal display, a plasma display, or the like, the organic EL display using the organic EL device is excellent in terms of thin device implementation, a high-speed responsibility, characteristics of viewing angle, and the like. Recently, a flexible display using a plastic substrate has been developed. As a driving method of the organic EL display, there are a passive matrix type and an active matrix type.
In the passive matrix type, positive and negative electrodes of the organic EL device as interdigitated electrodes are disposed in the X and Y directions; the one electrode is used as a scan electrode; and the other electrode is used as a data electrode. Light is emitted by applying a voltage from an external constant current circuit to the pixels at the intersections. Since the thin film transistor for driving the organic EL device is unnecessary, the passive matrix type has an advantage in terms of production cost in comparison with the later-described active matrix type. However, since the number of scan electrodes is increased as the number of pixels in the display screen is increased, the duty ratio of driving the pixel is decreased. Therefore, there is a limitation in that high luminance may not be obtained.
In the active matrix type, turning ON and OFF of the thin film transistor (TFT) is performed for each pixel, so that the lighting state is maintained by holding capacitance (condenser). Therefore, high luminance can be sustained although the number of pixels is increased. Accordingly, the active matrix type can be used for the applications such as TVs where the number of pixels is large.
In an active matrix type liquid crystal display, one transistor for selecting pixels is sufficient. However, in the case of an active matrix type organic EL display, at least two TFTs are needed. In other words, besides the transistor for selecting the pixels, a transistor for allowing a current to flow in the organic EL device of the selected pixel and allowing the organic EL device to emit light is needed. Therefore, if an aperture ratio of the display is considered, in the case of the organic EL display, the size of the TFT becomes an important problem in comparison with the case of a liquid crystal display. If the size of the TFT is small, the aperture ratio of the display can be increased.
The TFT using an oxide thin film as the active layer is transparent with respect to visible light, so that it can be expected that the aperture ratio of the display is increased.
As TFTs used to an active matrix type, an a-Si TFT using an amorphous silicon (a-SI) as the active layer and a low-temperature p-Si TFT using a low-temperature polysilicon (low-temperature p-Si) as the active layer are put to practical use and widely used as a liquid crystal display (refer to JP 2008-59824 A).
In this manner, in order to implement a large-size, high-accuracy organic EL display using an organic EL device, an active matrix driving type needs to be selected as described above. In this case, if an a-Si TFT is used as the TFT, since the field effect mobility is about 0.5 cm2/Vs, in the case where the area of the pixel is large or the case where the number of scan electrodes is large, for example, 2,000 or more, there is a problem in terms of high-speed responsibility and high luminance. In other words, in the case where the size of the pixel of the organic EL device is large, in order to flow a sufficient current, the size of the TFT needs to be large. However, the aperture ratio of the pixel is decreased, so that high luminance may not be implemented. In addition, in the case where the number of scan electrodes is large due to high accuracy, as the number of scan electrodes is increased, the writing time is shortened. Therefore, a sufficient time for charging the holding capacitance may not be secured, so that turning on of the TFT may not be satisfactorily performed.
In addition, in the a-Si TFT, a change in the reverse voltage Vt due to current stress is large, so that the occurrence of unbalance of the driving current is inevitable for a long time of the driving. In the organic EL device, the unbalance of the driving current leads to irregularity in luminance.
On the other hand, in the case where a low-temperature p-Si TFT is used, the mobility is in a range from 50 cm2/Vs to 150 cm2/Vs, so that the TFT can be sufficiently adopted to the driving of a large-size, high-accuracy organic EL display. In addition, since a change in Vt due to the current driving is smaller by two digits or more than that of the a-Si, there is no problem.
However, in the manufacturing of the low-temperature p-Si, in order to perform molten crystallization of a silicon film, excimer laser beams are needed; and in the case of a large-size display, an excimer laser beam having a length corresponding to a screen width is needed. In the current state, the maximum length of the laser beam is 465 mm, and thus, a display having a width more than the maximum length may not be manufactured by using a low-temperature p-Si.
In addition, with respect to the low-temperature p-Si TFT, since the manufacturing process temperature is high, from 500° C. to 600° C., the plastic substrate may not be used. Therefore, it is difficult to implement a flexible display.